Decoder and decoding method for decoding a code by selecting a path out of paths in a trellis diagram

1. A decoder that decodes a code by selecting, based on a predetermined condition, a path out of paths representing a transition of each of a plurality of states in a trellis diagram, the decoder comprising: a storing unit that stores a value of a first bit of bits representing a state of a source of the transition in the trellis diagram as information on a selection history of a path selected before time (k−(a constraint length of a code)+1) by adding the value to path selection history information on paths selected by time k−1 when a path at time k is selected; a path detecting unit that detects a path to be excluded from a path selection candidate, based on the information stored in the storing unit and information on a state of a transition source when a state transition occurs between time k−1 and time k; and a decoding unit that decodes a code by deleting the path detected by the path detecting unit.

2. The decoder according to claim 1 , wherein the path detecting unit performs coincidence judgment for each bit between a bit string in the information stored in the storing unit and a specific bit string, and detects the path to be excluded from the path selection candidate based on a result of the coincidence judgment.

3. The decoder according to claim 2 , wherein the specific bit string is a bit string in which bits are consecutively reversed.

4. The decoder according to claim 2 , wherein the specific bit string is a bit string in which identical bits are consecutive.

5. The decoder according to claim 1 , wherein the path detecting unit detects the path to be excluded from the path selection candidate by counting bits of a bit string in the information stored in the storing unit.

6. The decoder according to claim 5 , wherein the path detecting unit counts bits that are consecutively reversed in the bit string.

7. The decoder according to claim 5 , wherein the path detecting unit counts identical bits that continue in the bit string.

8. The decoder according to claim 1 , further comprising: a path selecting unit that includes the path to be excluded from the path selection candidate in the path selection candidate by changing a path that leads to the path detected by the path detecting unit, which has been selected before, and executes selection of a path.

9. The decoder according to claim 8 , further comprising: a likelihood-information generating unit that generates, when the selection of a path is executed by the path selecting unit, likelihood information of a state of a transition destination of a selected path, based on likelihood information concerning a decoding result at a point in a past in a state of a transition source of the path.

10. A method of decoding a code by selecting, based on a predetermined condition, a path out of paths representing a transition of each of a plurality of states in a trellis diagram, the method comprising: storing a value of a first bit of bits representing a state of a source of the transition in the trellis diagram as information on a selection history of a path selected before time (k−(a constraint length of a code)+1) by adding the value to path selection history information on paths selected by time k−1 when a path at time k is selected; detecting a path to be excluded from a path selection candidate, based on the information stored at the storing and information on a state of a transition source when a state transition occurs between time k−1 and time k; decoding a code by selecting the path detected by the detecting.